This application is based on Application No. 2001-131953, filed in Japan on Apr. 27, 2001 and Application No. 2001-341797, filed in Japan on Nov. 7, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a solid-state light source apparatus, and particularly to a solid-state light source apparatus used for a terahertz band spectroscopic light source, an imaging light source, a light source for communication, and a light source for measurement.
2. Description of the Related Art
As a light source for generating a terahertz band beam, although there was a GaAs photoconductive device, a magnetic field application type semiconductor device, an optical parametric oscillator using LiNbO3, a difference frequency generation device using an organic nonlinear optical crystal, or the like, all of them had low efficiency and low output power.
Since a conventional semiconductor pseudo phase matching device using diffused junction has high scattering at a junction interface, it falls far short of practical use, and naturally, there was no terahertz light source using this technique.
A conventional solid-state light source apparatus will be described with reference to the drawing. FIG. 5 is a view showing the structure of a conventional solid-state light source apparatus disclosed in, for example, xe2x80x9cLaser Research, Vol. 26, No. 7, p. 515 to 521, July 1998xe2x80x9d. FIG. 5 is the structural view of an example of a photoconductive device used for terahertz wave generation.
In FIG. 5, reference numeral 100 designates a photoconductive device; 101, a semiconductor substrate; 102, a photoconductive thin film; 103, parallel transmission lines; 104, a dipole antenna; 105, a gap; 106, a direct current power source; 110, an optical pulse; and 111, a terahertz electromagnetic wave.,
In this photoconductive device 100, the parallel transmission lines 103 made of transmission lines 103a and 103b are formed on the substrate 101 of a high speed response semiconductor such as GaAs and the photoconductive thin film 102 of low temperature growth GaAs or the like, and a single optical switch made of the minute dipole antenna 104 is provided at the center portion.
The minute gap 105 with several xcexcm, for example, exists at the center of the optical switch 104, and a suitable voltage is applied to the gap 105 by the direct current power source 106.
When a laser beam having energy higher than the band gap of the semiconductor enters on the gap 105 as the optical pulse 110, free carriers are generated in the semiconductor, a pulse-like current flows, and the terahertz electromagnetic wave 111 in proportion to the time differential of the pulse-like current is generated.
Thus, the terahertz electromagnetic wave 111 is generated when the pulse-like current is, for example, on a picosecond level or less, and further, it is generated when a short pulse laser beam in which the optical pulse 110 is on a picosecond level or less is irradiated.
As disclosed in xe2x80x9cLaser Society Scientific Lecture Meeting, 17th Annual Conference, 23aII4, p. 194 to 197xe2x80x9d, two continuous-wave laser beams are optically mixed with each other on a photoconductive device, so that a terahertz wave can be continuously generated. When two monochromatic beams with different frequencies are mixed, a resultant amplitude is modulated by a difference frequency. When the mixed wave (light beat) is irradiated to the photoconductive device, a photocurrent is modulated, and an electromagnetic wave corresponding to the difference frequency is radiated from an antenna. Thus, when the frequencies of the two continuous-wave laser beams are adopted so that the difference frequency becomes about terahertz, the terahertz wave is generated.
As disclosed in xe2x80x9cLaser Research, Vol. 26, No. 7, p. 527 to 530, July 1998xe2x80x9d, when a light pulse of picosecond or less as a laser beam is irradiated to a semiconductor such as InAs or GaAs put in a magnetic field, a terahertz wave can be generated.
Further, as disclosed in xe2x80x9cLaser Research, Vol. 26, No. 7, p. 522 to 526, July 1998xe2x80x9d, LiNbO3 is used as a crystal having a secondary nonlinear optical effect, and light waves are caused to enter upon the crystal, and an optical parametric oscillator is constructed under phase matching conditions, so that a terahertz beam can be generated.
As disclosed in xe2x80x9cOPTICS LETTERS, Vol. 25, No. 23, pp. 1714-1716, 2000xe2x80x9d, an organic crystal with high nonlinearity is used as a crystal having a secondary nonlinear optical effect, two laser beams with a difference frequency of terahertz enters upon the crystal, and difference frequency generation is carried out under phase matching conditions, so that a terahertz beam can be generated.
Further, as disclosed in xe2x80x9c61th Applied Physics Society Scientific Lecture Meeting, Collection of Lecture Preparatory Papers, No. 3, 4a-L-8, p957, 2000xe2x80x9d, a bulk type semiconductor material is used as a material having a secondary nonlinear optical effect, two laser beams with a difference frequency of terahertz are caused to enter on the nonlinear material, and difference frequency generation is carried out under phase matching conditions, so that a terahertz beam can be generated.
However, the foregoing prior art had problems as follows:
In the generation of the terahertz beam by the photoconductive device using the excitation of the short pulse laser beam, the efficiency was low and the output power was low. Further, since the line width was wide, a light source of a single longitudinal mode did not exist as well.
In the generation of the terahertz beam by the photoconductive device using the excitation of the two continuous-wave laser beams, the efficiency was low and the output power was low.
In the generation of the terahertz beam by the semiconductor device put in the magnetic field using the excitation of the short pulse laser beam, the efficiency was low and the output power was low. Besides, since the line width is wide, a light source of a single longitudinal mode did not exist as well.
In the generation of the terahertz beam by the optical parametric oscillator using LiNbO3 as the nonlinear optical device, the absorption of the terahertz beam in LiNbO3 was large, the extraction efficiency of the generated terahertz beam was low, and the output power was low. Further, since the output angle of the terahertz beam was not coincident with the optical axis of the exciting beam, in the optical parametric oscillator, it was difficult to take a long interaction length between the exciting beam and the terahertz beam obtained by wavelength conversion, and the wavelength conversion had low efficiency and the output power was low.
In the generation of the terahertz beam by the difference frequency using the organic crystal as the nonlinear optical device, the efficiency was low and the output power was low.
Besides, in the generation of the terahertz beam by the difference frequency using the bulk type semiconductor material as the linear optical device, since it was difficult to take a long distance in the phase matching conditions, the efficiency was low and the output power was low.
Moreover, in the conventional semiconductor pseudo phase matching device using diffused junction, there were also problems that since scattering at the junction interface was high, it falls far short of practical use, and naturally, there was no terahertz beam source using this technique.
The present invention has been made to solve the foregoing problems, and a pseudo phase matching difference frequency generation device by diffused junction of semiconductors is used to generate a terahertz wave. Since the semiconductor such as GaP or GaAs has a large nonlinear optical constant, it is suitable for high efficiency wavelength conversion, and is transparent in a terahertz region. Further, the semiconductor has large thermal conductivity and is also suitable for high power operation. Further, when a tunable laser of a band of 1 xcexcm is used as a difference frequency light source, a diffused junction period for generating the terahertz wave by pseudo phase matching difference frequency generation is very long, for example, several mm, and the number of junction interfaces can be suppressed to the minimum, so that a low-loss device can be fabricated. Further, an object of the invention is to provide a terahertz wave light source which can tune a terahertz generation wavelength over several hundred up by merely adjusting device temperature and changing the wavelength of one of the difference frequency light sources slightly by the order of nm.
According to a first aspect of the invention, a solid-state light source apparatus includes a first excitation laser light source for outputting a laser beam of a first wavelength, a second excitation laser light source for outputting a laser beam of a second wavelength, a difference frequency between the laser beam of the first wavelength and the laser beam of the second wavelength being in a terahertz band, and a nonlinear wavelength conversion device which is disposed at a place where a first optical axis of the laser beam of the first wavelength overlaps with a second optical axis of the laser beam of the second wavelength, and generates a terahertz beam in a direction coaxial with the first and second optical axes on the basis of irradiation of the laser beams of the first and second wavelengths.
A solid-state light source apparatus according to a second aspect of the invention is such that the first excitation laser light source is a fixed wavelength laser, and the second excitation laser light source is a tunable wavelength laser.
A solid-state light source apparatus according to a third aspect of the invention is such that the first excitation laser light source is a Nd:YAG laser, and the second excitation laser light source is a Yb:YAG laser.
A solid-state light source apparatus according to a fourth aspect of the invention is such that the first excitation laser light source outputs a monochromatic beam of the first wavelength of 1.064 xcexcm and the second excitation laser light source outputs a monochromatic beam of the second wavelength in a band of 1 xcexcm.
A solid-state light source apparatus according to a fifth aspect of the invention is such that the first and second excitation laser light sources are Yb:YAG lasers.
A solid-state light source apparatus according to a sixth aspect of the invention is such that the first excitation laser light source is a Nd:YLF laser, and the second excitation laser light source is a Yb:YAG laser.
A solid-state light source apparatus according to a seventh aspect of the invention is such that the nonlinear wavelength conversion device is a semiconductor pseudo phase matching device in which a plurality of first semiconductor materials each having a length of xcex9/2 in a direction coaxial with the first and second optical axes and a plurality of second semiconductor materials each having a length of xcex9/2 are united with one another by diffused junction, and the first and second semiconductor materials are disposed so that directions vertical to the first and second optical axes are [001] axes, directions of the respective [001] axes are parallel to one another, and the directions of the [001] axes are alternately inverted.
A solid-state light source apparatus according to an eighth aspect of the invention is such that the first and the second semiconductor materials of the semiconductor pseudo phase matching device are transparent materials in a terahertz region.
A solid-state light source apparatus according to a ninth aspect of the invention is such that the first and the second semiconductor materials of the semiconductor pseudo phase matching device are made of GaP.
A solid-state light source apparatus according to a tenth aspect of the invention is such that the first and the second semiconductor materials of the semiconductor pseudo phase matching device are made of GaAs.
A solid-state light source apparatus according to an eleventh aspect of the present invention is such that the nonlinear wavelength conversion device is a semiconductor pseudo phase matching device in which a plurality of first semiconductor materials each having a length of xcex9/2 in a direction coaxial with the first and second optical axes and a plurality of second semiconductor materials each having a length of xcex9/2 are united with one another by diffused junction, and the first and second semiconductor materials are disposed so that azimuths of the first and second semiconductor materials are the same as a direction perpendicular to the first and second optical axes and directions thereof are alternately inverted.
A solid-state light source apparatus according to a twelfth aspect of the present invention is such that the first and second semiconductor materials of the semiconductor pseudo phase matching device are transparent materials in a terahertz region.
A solid-state light source apparatus according to a thirteenth aspect of the present invention is such that the first and second semiconductor materials of the semiconductor pseudo phase matching device are GaP.
A solid-state light source apparatus according to a fourteenth aspect of the present invention is such that the first and second semiconductor materials of the semiconductor pseudo phase matching device are GaAs.